Using inverter type motor drive devices, electrical motors can be controlled more freely. Either torque or speed can be controlled using such devices.
The motor drive device can receive a reference torque as an input. To provide a complete control loop, an estimate of an actual torque of the controlled motor can be provided as a feedback variable.
The accuracy of the actual torque estimate is typically not very high due to that rotor flux varies with rotor temperature and between individual motors. This leads to reduced ability to control torque, power flow, DC voltage stability and engine point of operation. These are disadvantages in a typical hybrid system with electrical machines, combustion engine and dc link.